1. Field of the Invention
The present invention relates to an optical transmission system with a radio technique merged with an optical communication technique, using an optical fiber line between a control station which transmits a radio signal to a plurality of base stations and the base stations which transmit the radio signal to terminal stations. This application is a counterpart application of Japanese application Serial Number 186518/2002, filed Jun. 26, 2002, the subject matter of which is incorporated herein by reference.
2. Description of the Related Art
As the optical transmission system with the radio technique merged with the optical transmission technique, the following are well known:
Tsukamoto et al., “Current Activities and Future Trends in Radio-over-Fiber Network”, Journal of IEICE (Institute of Electronics, Information and Communication Engineers), Vol. 80, No. 8, pp. 859–868, August 1997; and
Tarusawa et al., “Single Fiber Multi Terminal Optic Links for Mobile Radio Communication using Automatic Wavelength-Offset Control”, Shingakugiho, RCS94-70, pp. 7–12, September 1994.
Nowadays, following the rapid spread of mobile communications, the introduction of a micro-cell system has progressed with a view of securing frequency band and making portable equipment small in size. According to the micro-cell system, a micro-cell zone having a small zone radius is provided so as to improve the utilization efficiency of radio frequency. To provide wideband service, many radio base stations are required. On the other hand, the radio frequency is moving to millimeter wave band so as to secure wideband frequency. However, it is difficult to realize long-distance signal transmission because of the high attenuation of signals in space. In these circumstances, a communication technique with radio waves merged with light, or photonics which exhibits advantages of both optical communication and radio communication has been considered.
As for the mobile communication system into which the micro-cell method has been gradually introduced, a method for transmitting signals using the optical fiber communication technique which is characterized by wide area coverage and wideband has been considered. An optical fiber connects a base station to a control station. A radio signal received by the base station is transmitted to the control station while the radio form of the signal is maintained. Control functions are collectively provided at the control station, whereas it suffices that the base station has only a function of converting a radio signal to an optical signal and a function of converting an optical signal to a radio signal. Therefore, a small-sized system can be provided at low cost.
Further, in order to improve optical fiber utilization efficiency while making use of the low transmission loss of the optical fiber, an optical fiber link method using one optical fiber for each of an up link and a down link and dependently connecting many base stations has been considered.
The configuration of the optical transmission system which employs a method for dependently connecting many base stations to an optical fiber connected to a control station will be described. In the optical transmission system for a down link, the control station consists of a radio signal transmitter and an electrical-to-optical converter (E/O converter). The E/O converter converts a radio signal with a frequency fd to an optical signal, and transmits the converted optical signal to the optical fiber. A plurality of (n) radio base stations are dependently connected to the optical fiber. Each radio base station consists of a photodiode (PD), a radio signal amplifier (RF-AMP), and an antenna.
The optical signal transmitted by the optical fiber is distributed by an optical coupler and input into the PD. The PD, which acts as an optical-to-electrical converter (O/E converter), converts the optical signal to a radio signal. The radio signal is amplified by the RF-AMP to have desired power and the resultant radio signal is output from the antenna to the air. The mobile terminal receives the radio signal thus output.
The optical coupler used herein is to distribute optical power to a plurality of base stations. If the coupling factor of the optical coupler is, for example, 0.1, the optical power distributed to branched base stations is 10% and the remaining 90% is output to the transmission optical fiber as it is. To make the optical power distributed to the respective base stations equal, the optical power distributed to each base station is set at (output from the control station×1/the number of base stations). As the number of base stations increases, the coupling factor lowers.
In addition, regardless of the number of base stations, the optical power distributed to the first base station is set at (output from the control statio×1/the number of base stations). The coupling factor of the optical coupler which distributes the optical power to the terminal base station and the base station just before the terminal base station is 0.5. Thus, the number of the necessary coupling factors of the optical couplers is (the number of base stations—1) between (1/the number of base stations) and 0.5. If the number of base stations increases, it is necessary to provide optical couplers having coupling factors fragmented according to the increased number of base stations.
It is assumed herein that, for example, ten base stations are dependent on the down link of the optical transmission system. If so, to deliver equal optical power to the respective base stations, the optical power delivered to each base station is set at one-tenth of the optical power output from the control station. Further, the PD-side coupling factor of each optical coupler is required to have coupling factor accuracy as shown in Table 1 below. To simplify calculation, it is assumed herein that there is no transmission fiber loss and no optical coupler coupling loss.
TABLE 1No.123456789Coup-0.1000.1110.1250.1430.1670.2010.2510.3350.500lingfactor
A plurality of base stations are thus dependently connected to one optical fiber using optical couplers. In this case, if the number of base stations increases, the optical power which can be delivered to each base station disadvantageously lowers and the coupling factor of each optical coupler is disadvantageously required to have higher accuracy. Further, if a base station connected to this down link optical fiber is added, all the base stations should change their optical couplers to those having newly designed coupling factors. Therefore, it is disadvantageously impossible to easily increase the number of base stations.